So in the "Cayde's Stash" mission, you can go into a secret room that Cayde had, and there's some cool dialogue.
He says that the hearts in his deck of cards stood for a girl he loved. When pressed about it, he uncomfortably avoids the question.
Could the girl have been The Stranger? I think either that or maybe Amanda Holliday. But probably The Stranger.
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on the grimoire card Ghost Fragment: Rasputin 2, it says the following: "She hunts the Valus named Ta'aurc by the grunting radio traffic of his bodyguards. Cayde sent her to Mars to track and so track she will even if it kills her a hundred times. For him she will hunt forever." So with that information she would probably be a guardian...but yeah I would like her to be The Stranger!